This invention relates to solid lubricants for metals, metal alloys and semiconducting materials. The invention is particularly useful in applications such as automotive transport, aircraft industry, space technology or ultra-high vacuum.
Following carbon fullerenes and carbon nanotubes (Iijima S, Helical microtubules of graphitic carbon, Nature 354, 56-58 (1991); Kroto H W et al., C60: Buckminsterfullerene, Nature 318, 162-163 (1985)) hollow nanoparticles and nanotubes of metal dichalcogenides, boron-carbides and other layered compounds have been synthesized as a single phase in recent years (Chopra N G, et al., Boron nitride nanotubes, Science, 269, 966-967 (1995); Feldman Y, et al., High-rate, gas-phase growth of MoS2 nested inorganic fullerenes and nanotubes, Science, 267, 222-225 (1995); Rothschild A, et al., The growth of WS2 nanotubes phases J An. Chem. Soc, 122, 5169-5179 (2000); Tenne R, et al., Polyhedral and Cylindrical Structures of WS2. Nature 360: 444-445 (1992)). These materials were designated under the generic name inorganic fullerene-like materials (IF).
The tribological properties of solid lubricants such as graphite and the metal dichalcogenides MX2 (where M is molybdenum or tungsten and X is sulphur or selenium) are of technological interest for reducing wear in circumstances where liquid lubricants are impractical, such as in space technology, ultra-high vacuum or automotive transport. These materials are characterized by weak interatomic interactions (van der Waals forces) between their layered structures, allowing easy, low-strength shearing.
Solid lubricants are required to have certain properties, such as low surface energy, high chemical stability, weak intermolecular bonding, good transfer film forming capability and high load bearing capacity. Conventional solid lubricants such as MoS2 particles, graphite, and polytetrafluoroethylene (PTFE) have weak interlayer bonding which facilitate transfer of said materials to lo the mating surface. Such transfer films are partially responsible for low friction and wear.
The use of metal dichalcogenides and MoS2 particles as solid lubricants in various applications, is well documented (Singer I L, in Fundamentals of Friction: Macroscopic and Microscopic Processes (eds. 3. Singer I L and Pollock H M), p. 237 (Kluwer, Dordrecht, 1992)). Recently, the tribological applications of hollow nanoparticles of WS2 as an additive for lubrication fluids, has also been demonstrated (Rapoport L, et al., Hollow nanoparticles of WS2 as potential solid-state lubricants, Nature, 387, 791-793 (1997).
It is an object of the present invention to develop new composites of metal, metal alloy or semiconducting material, providing high durability and mechanical strength.
The above object is achieved by the present invention, which provides new composite materials for use to reduce friction coefficient and wear rates and for increasing the load bearing capacity of parts made of such materials. The new composite materials of the invention comprise a porous matrix made of metal, metal alloy or semiconducting material and hollow fullerene-like nanoparticles (IF) of a metal chalcogenide compound or mixture of such compounds, said composite materials having a porosity between about 10% and about 40%.
The present invention also provides a method for preparing the new composite materials of the invention.
The IF nanoparticles used in the composite materials of the invention have a diameter between about 10 and about 200 nm. In view of their small sizes, these nanoparticles can be impregnated into highly densified matrices.
Without being bound to theory, it is suggested that the IF nanoparticles are impregnated into the pores of the porous matrix and are slowly released to the surface, where they serve as both lubricant and spacer. The behavior of IF nanoparticles is compared hereinafter with commercially available WS2 and MoS2 platelets with 2H polytype structure (2H).